Updated

The cause of medical helicopter crash Thursday night that killed all four people aboard in Central California remains a mystery, authorities said.

The pilot was Thomas Hampl, 49, the nurse was Marco Lopez, 42, and the paramedic was Kyle Juarez, 37, said Todd Valeri, president of the helicopter's owner and operator. Authorities have not released the name of the patient of said why she was being taken to the hospital.

The thick fog and heavy rain at the time are at the forefront of the investigation, but federal authorities are still gathering data on that and other possible causes including mechanics and the pilot's history, Joshua Cawthra of the National Transportation Board said at a briefing Friday.

The NTSB is leading the investigation of the crash.

He said there were no communications with air traffic controllers or dispatchers in the moments before the Bell 407 helicopter went down.

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"Hopefully at the end we'll be able to say what happened, why it happened and ultimately prevent this type of accident from happening again," Cawthra said.

The helicopter was staffed by a "seasoned crew," said Dan Lynch, EMS Director for Fresno, Kings, Madera and Tulare counties. "They had been a team for quite some time."

The helicopter was headed from Porterville Municipal Airport south to San Joaquin Community Hospital in Bakersfield when it crashed about halfway through the 50-mile trip, Kern County fire officials said.

The helicopter went down amid rolling hills of cattle-grazing country east of the town of McFarland, 135 miles northwest of Los Angeles. The wreckage was 2½ miles from the nearest highway. Kern County sheriff's deputies secured a road leading to the site.

The helicopter's cabin was fairly intact but the tail was broken off and lay 30 yards away while debris was scattered over a 50-square-yard area, according to a description by sheriff's spokesman Ray Pruitt.

Air Ambulance's Skylife Air Medical service operates three helicopters out of the Fresno and Visalia airports. Valeri said that SkyLife has never had an aircraft crash since it partnered with Rogers Helicopters Inc. in 1991. The air ambulance transports about 1,000 patients a year, he said.

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